Defining standard atmosphere
An ideal understanding of an environment that is anything but ideal.
A standard density altitude chart translates pressure altitude into density altitude by accounting for temperatures that may differ from the ISA standard.
One byproduct of the rising air is lowered surface pressure. In fact, beneath the ITCZ, the surface pressure is usually well below Earth's average of 29.92 inches Hg (1013.25 hPa).
So, then, why should an airport so near the Equator be reporting an altimeter setting more than an inch higher than reality? Wouldn't this present a serious hazard for aircraft, placing them well below their expected altitude at a high-altitude airport in mountainous terrain? The answer is no, not really, and here's why.
An altimeter setting, or QNH, is really just the absolute surface pressure at the airport (QFE) corrected to sea level using the equation that covers pressure in the standard atmosphere. We must remember that the atmosphere at any place is very rarely standard, and this is especially true in places that are routinely warmer or colder than the average for the planet.
In the tropics, the warmer air means a lower than average density of air at any given altitude in the lower atmosphere as well as above-ISA temperature at that altitude. When a station pressure at 8361 ft MSL of, say 21.80 (slightly below the ISA average for that height) is plugged into the equation for the standard atmosphere (without accounting for the higher temperature), the "correction" produces a sea-level value of 31.30—much higher than it really is.
In reality, a closer correction for sea-level pressure can be obtained by factoring in the difference between the observed temperature and the ISA estimated temperature. However, if this were done it would create a hazard to aircraft.
The reason for this is rather straightforward—the pressure altimeters we use in our aircraft rely on the same equation, but don't accommodate any deviation from ISA temperature.
So, even though we are dialing a fictitious value into our Kollsman windows, the altimeter interprets it correctly to give you your actual altitude relative to the true field elevation, keeping you clear of the terrain. Next time you fly into a high-altitude tropical airport, see if your pressure altimeter agrees with your radio altimeter or GPS. With the reported altimeter setting dialed in, they should be pretty close.
Similarly, consider a high-latitude, high-altitude airport, such as YBA (Banff AB, Canada) at 4583 ft). Especially in the winter, where the cold air compresses in the lower atmosphere, the QFE at the airport would be equivalent to a pressure found much lower in the standard atmosphere.
When the corrective equation is applied, the apparent elevation change to sea level is much smaller, so the QNH is actually underestimating the sea-level pressure—meaning that you might see an altimeter setting of 29.90 (1012.5 hPa) even though the region is sitting under an intense winter high pressure of 30.60 inches Hg (1036.2 hPa). Again, since the pressure altimeter uses the same correction, the absolute elevation will be correct.
ISA and performance
Atmospheric pressure is a product of both the density of air molecules at a given altitude and the air temperature. On average, it is 29.92 in Hg (1013.25 mb or hPa) at sea level and in the lower atmosphere can be reasonably estimated by a linear decrease. Transient pressure cells or non-standard temperatures necessitate corrections to ISA pressure altitudes.
So, if the "standard" atmosphere almost never exists, why do we use it? The answer is performance. From the Piper Cub to the Boeing 787, engineers must be able to determine how well the aircraft will perform under various atmospheric conditions before it ever takes to the skies. For the pilot, the calculation of that performance must be transferable from one aircraft to the next.
While air temperature has no effect on setting the aircraft's pressure altimeter to QNH, it plays a very significant role in determining the ability of the aircraft to produce both lift and thrust.
These forces are a function of the density of the air through which the aircraft is moving. As air density is lowered, there are fewer molecules to exert a lifting force, and fewer to facilitate combustion inside the engine. With each incremental decrease in air density, the performance of the aircraft is diminished until its wings and engines are no longer capable of climbing or even maintaining altitude.
Designers can calculate exactly at what air density a certain performance can be expected, and that density can in turn be translated into an altitude using the ISA. In this way, service ceilings, power output, acceleration, climb rates and runway distances can be estimated before they are established during flight testing.
The performance values given in the aircraft's operating manual are based on pseudoaltitudes. This idea of a pseudoaltitude is often referred to as the altitude the aircraft "thinks" it is at. There are 2 of these pseudoaltitudes with which a pilot should be familiar—pressure and density altitude.
Pressure altitude is the altitude that would be estimated by correcting ISA sea-level pressure (QNE) of 29.92 (1013.25 hPa) to the elevation of the aircraft above mean sea level. It is the same correction that's used when flying the flight levels, and bases the elevation on pressure relative to the standard atmosphere, rather than any localized barometric pressure variation.
Because using actual altitude is meaningless to an aircraft that relies on air density to fly, pressure altitude is what is used in all aircraft performance calculations. However, because the real atmosphere is almost never standard, pilots need to know how to adjust those fixed performance numbers to accommodate deviation from the ISA.
This correction comes in the form of density altitude—a term which describes the actual altitude at which the air density matches a theoretical ISA altitude, when adjusted for actual temperatures that are warmer or colder than the standard.
Some aircraft performance charts allow you to estimate performance values by including temperature offsets to the pressure altitude. Others require you to use the density altitude in place of the pressure altitude values. Either way, you will need to adjust for temperature in order to obtain accurate performance values from the aircraft's charts.
If you know the pressure altitude and temperature for a location, estimating the density altitude is fairly straightforward. First, determine the difference between the air temperature and ISA (OAT–ISA). Then, if you are working in feet and degrees Fahrenheit, your density altitude is approximately the temperature difference in degrees Fahrenheit multiplied by 67 and that result added to the pressure altitude in feet.